Acute renal failure (ARF) is a general term referring to conditions resulting from an acute insult to the microvasculature of the kidney. Although the exact nature of the acute insult may vary, ARF is generally characterized by a sudden decline in glomerular filtration rate, the accumulation of nitrogen wastes and the inability of the kidney to regulate electrolyte and water balance.
Despite the technical advances in the care of patients who suffer from ARF and improvements in the understanding of the pathophysiology of the disease progress, there is still a high mortality associated with this condition. This is especially true in the ICU setting where ARF is associated with a mortality of 50-90%. Although numerous experimental models have shown that a variety of agents, including dopamine, osmotic agents, atrial natriuretic peptide, insulin like growth factor and endothelial receptor antagonists are effective in animal models, these agents have been ineffective in clinical studies of ARF.
Thrombomodulin (TM) is a glycoprotein anchored on the membrane surface of endothelial cells on many organs, including lung, liver, and kidney and has been shown to possess well established roles in inflammation, fibrinolysis, apoptosis, cell adhesion and cellular proliferation. TM can complex with thrombin to inhibit thrombin's pro-coagulant activity and once complexed, the TM/thrombin complex can enhance the activation of Protein C (PC) 1000 fold. Activated PC (APC) also modulates the coagulation cascade in addition to possessing anti-inflammatory properties.
TM is composed of five domains: an N-terminal lectin-like binding domain, an epidermal growth factor (EGF) domain which consists of 6 EGF-like repeats, a Ser/Thr-rich region, a transmembrane domain and a cytoplasmic domain. Soluble TM variants have been constructed by deleting the cytoplasmic and transmembrane domains. To attempt to understand TM function, additional TM variants have been generated. TM deletion variants have been used to determine the smallest TM fragment (EGF-like repeats 4-6) which retains thrombin binding and PC activation. Nagashima and colleagues used alanine scanning of this TM region to determine residues which are critical for thrombomodulin activity. Changes of specific residues to alanine within polypeptides consisting of EGF 4-6 resulted in sTM variants with greatly reduced levels of PC activation. However, the use of sTM or any sTM variant in the treatment of acute renal failure was not disclosed. (Nagashima et al, Journal of Biological Chemistry, 268(4): 2888-2892, 1993).
Recent results indicate that APC is effective in preventing ischemia/reperfusion induced renal injury in rats. (Mizutani et al., Blood, 95(12): 3781-3787, 2000). The study indicates that APC may prevent this renal injury by inhibiting leukocyte activation as well as by attenuating coagulation. Although the use of APC appears promising, the risks of inducing an anti-coagulation environment in an acute renal context are not known.
There remains a need for an alternative option for the treatment of acute renal failure that is both safe and effective. Accordingly, Applicants have discovered that soluble thrombomodulin variants, that do not activate Protein C, are unexpectedly effective in in vivo experimental models of ARF. Thus, using a soluble thrombomodulin variant which may not modulate the coagulation cascade offers a potentially significant alternative approach to the prevention and treatment of ARF.